The present invention relates to a buffer tank for supplying an extrusion device with a polymer melt in a substantially constant manner, the buffer tank comprising at least one polymer melt entry opening, at least one polymer melt exit opening, and at least one compensation chamber that is arranged between polymer melt entry opening and polymer melt exit opening and has the polymer melt flowing therethrough, and whose volume is variable in dependence upon a pressure prevailing in the polymer melt.
Such buffer tanks are generally used in extrusion devices which must be fed with a mostly high-viscosity polymer melt at a constant pressure, if possible. A special use of such buffer tanks for extrusion devices is encountered in the textile industry in the field of spinning systems and spinning machines, where the polymer melt is a spinning solution consisting of cellulose, amine oxides, such as N-methylmorpholine-N oxide (NMMO), and water. The spinning solution is spun by the spinning machines into yarns. Other applications of the buffer tank can be found in blow molding machines and in deep-drawing and injection-molding machines.
In spinning machines the quality of the yarns essentially depends on a constant supply of the polymer melt to the spinning machine. To ensure that the spinning machine is constantly fed with polymer melt, buffer tanks are used to compensate for variations in volume flow and pressure in the supply line of the polymer melt by varying the volume of a compensation chamber in the buffer tank in dependence upon a volume flow and/or pressure variation in the polymer melt.
However, the use of a buffer tank creates new problems. The mechanical and chemical characteristics of the polymer melts, e.g. a cellulose solution, are time-dependent and vary in response to the respective residence time of the polymer melt in the buffer tank and in the supply line to the spinning machine. To obtain a uniform spinning quality, it must therefore be ensured that the polymer melt flows at a uniformly rapid speed through all areas of the buffer tank, if possible.
Problems arise when the polymer solution is a high-viscosity fluid and flows at relatively slow flow speeds through the buffer tank. On account of the low flow speed and the high viscosity, the flow of the polymer melt through the buffer tank is predominantly laminar.
The laminar flow is above all critical in view of the time dependence of the characteristics of the materials: On the one hand, the flow speed considerably drops in a laminar flow near walls; on the other hand, a laminar flow is very likely to separate. When a flow separates in the buffer tank, dead water zones or recirculation zones are formed. These two groups of problemsxe2x80x94irregular speed distribution and flow separationxe2x80x94effect different residence times of the polymer melt in the buffer tank. As a consequence, the chemical and mechanical characteristics of the xe2x80x9cagedxe2x80x9d polymer melt which flows through the areas of the buffer tank at a slower flow speed and thus remains in the buffer tank for a longer period of time are changed in comparison with a polymer melt flowing through the buffer tank at a fast speed.
The areas of separation of the polymer melt flow are flushed at least in part from time to time by the speed variations always observed during conveyance of the polymer melt, and because of the unsteady characteristics thereof. In the end, the polymer melt which has been trapped there for some time and has xe2x80x9cagedxe2x80x9d thus passes to the spinning machine and is spun in said machine, whereby the quality of the spun yarns varies considerably.
A measure for making the flow through the buffer tank uniform consists, according to WO 94/02408, in installing stirrers for actively transporting and mixing the flow through the buffer tank. Thanks to the mixing action the speed profile of the flow is made uniform and dead water and recirculation zones are avoided. However, the drive for driving the stirrers consumes a lot of energy, not least because of the high viscosity of the polymer melt. With an inaccurate design of the stirrer, there is the risk that the polymer melt heats up. Without expensive counter-measures, the chemical and mechanical properties of the polymer melt would change in such a case under the action of heat.
FR 2570323 relates to a device for supplying a low-viscosity elastomer at a predetermined constant throughput. The throughput is also to be achieved in cases where the supply with elastomer is briefly interrupted. The device is above all to be used for making connections for multiple glazings. In the device of FR 2570323, the elastomer is stored in a tank of a variable volume, a biased cylinder with a piston being arranged in the interior of the tank. In the stroke direction of the piston, a channel is passing therethrough for guiding the low-viscosity elastomer through the tank. A pump with two compression means is located at the outlet of the reservoir.
WO 96/05338 shows a buffer tank through which the polymer melt flows in axial direction. Behind the polymer melt entry opening, the compensation chamber is expanded in a diffusor-like manner. The compensation opening tapers towards the polymer melt exit opening in the manner of a nozzle. The volume prevailing between the diffusor section and the nozzle section can be increased or reduced in response to the necessary filling level in the polymer melt.
The polymer melt flow is very likely to separate because of the diffusor and the nozzle. Therefore, commercially available static mixer elements are installed in the compensation chamber of the buffer tank of WO 96/05338 for making the flow uniform and for preventing separation tendencies. The static mixer elements extend over the total cross-section of the compensation chamber.
The volume of the compensation chamber, however, must be increased considerably, so that the static mixer elements can be received therein. Therefore, the buffer tank with the static mixers may become too large for many applications. Moreover, on account of the long flow path and the large dimensions of the static mixer elements, the flow resistance of the buffer tank is increased many times, thereby increasing the amount of energy needed for conveying the polymer melt. This enhances the risk of a heating up of the polymer melt.
The buffer tank of WO 96/05338 has the drawback that the flow in the area of the diffusor and the nozzle tends to separate in cases where the angle of opening of diffusor and nozzle, respectively, becomes too large. This risk arises above all in the case of a slow flow of the high-viscosity polymer melt. In the separated flow, a separation whirl is formed, as well as dead water zones in which the high-viscosity polymer melt stagnates. Moreover, a new design and calculation of the opening angle are always needed for different flow conditions in different systems because the separating action depends on the speed of the flow.
In view of these drawbacks it is the object of the present invention to provide a buffer tank with improved fluid dynamics and to take measures for obtaining a uniform flow through the buffer tank, whereby the constructional size and the flow resistance of the buffer tank are not changed or only changed to an insignificant degree.
According to the invention this object is achieved for a buffer tank of the above-mentioned type in that the compensation chamber between the polymer melt entry opening and the polymer melt exit opening is divided into at least two partial chambers extending in the direction of flow.
Such a solution is simple and has the advantage that a diffusor-like enlargement and a nozzle-like narrowing of the flow cross-section can be dispensed with: Since the compensation chambers are divided into at least two partial chambers, the flow cross-section of the partial chambers can remain small so that a great discontinuous change in cross section is no longer required with respect to the polymer melt entry opening and the polymer melt exit opening. As a consequence, the tendency to form regions of separation is considerably reduced in the inventive design of the buffer tank in comparison with the prior art. The total flow cross-section formed by the two partial chambers is nevertheless relatively large, so that the flow resistance of the buffer tank is small. The constructional size of the buffer tank can be maintained in unchanged form.
According to a further advantageous development the discontinuous change in cross-section will become particularly small in cases where in a normal operative state of the buffer tank the flow cross-section of each partial chamber substantially corresponds to the flow cross-section of the polymer melt entry opening and/or the polymer melt exit opening. The buffer tank will then be in its normal mode of operation whenever the polymer melt is supplied to the spinning machine without trouble and the volume of the compensation chamber is substantially identical with the volume of the compensation chamber averaged over a long period of operation. Around this mean value the volume of the compensation chamber will vary upon a rise or drop in pressure and upon volume flow variations in the polymer melt.
For changing the volume of the compensation chamber a wall of the compensation chamber may be formed by a piston at least sectionwise. Said piston is movable in the buffer tank, and the volume of the compensation chamber is variable by its movement. In a further advantageous development a fluid-filled, i.e. a gas- or liquid-filled, piston chamber may be provided with a fluid supply line and a fluid discharge line at the side of the piston which faces away from the compensation chamber. Said piston chamber can be acted upon with an adjustable pressure through which the pressure in the compensation chamber, i.e. the pressure prevailing in the polymer melt, or the volume of the compensation chamber can be varied. Air can be discharged from the piston chamber via the fluid discharge line in case of a rise in pressure or volume flow in the polymer melt and an accompanying increase in the volume of the compensation chamber. Air is supplied via the fluid supply line into the piston chamber upon a decrease in volume of the compensation chamber.
Alternatively, or in addition to the fluid-filled piston chamber, a mechanical spring element may be provided for producing a spring force acting on the piston.
In a further development the piston may be connected to a displacement sensor by which a signal can be output. The volume of the compensation chamber can be calculated by the signal. This signal can be supplied to a data processing system for monitoring a spinning machine system, of which the buffer tank forms a part.
In a further advantageous development the compensation chamber may be designed as an annular chamber, and the piston accordingly as an annular piston.
When the compensation chamber is formed as an annular chamber, it is of advantage when the polymer melt entry opening and the polymer melt exit opening are arranged on diametrically opposite regions of the annular chamber. The annular compensation chamber is thus divided into two partial chambers of equal length that ensure a flow duration of equal length, independently of the partial chamber through which the polymer melt is flowing.
In a further advantageous development the polymer melt entry opening and/or the polymer melt exit opening can terminate in axial direction, i.e. in the direction of a symmetrical axis of the annular chamber, in the annular chamber. With such a design the buffer tank can easily be integrated into an existing pipe system. This design is in particular of advantage in combination with a piston if the polymer melt entry opening an/or the polymer melt exit opening are arranged in the compensation chamber to be opposite to the piston: Even with the smallest volume in the compensation chamber, the polymer melt entry opening and/or the polymer melt exit opening cannot be covered by the piston that is movable in the buffer tank.
Installation and removal of the buffer tank into and from an existing pipe system are easy if according to a further advantageous development the polymer melt entry opening and the polymer melt exit opening are arranged at the same side of the annular chamber. With this design the connections of the buffer tank are positioned at one side and are thus easily accessible.
The object underlying the invention is also achieved according to the invention for a buffer tank of the above-mentioned type by the measures that the compensation chamber is provided with at least one flow guiding element projecting into the flow of the polymer melt for optimizing the flow. The element is here smaller than the flow cross-section of the compensation chamber and makes the velocity profile uniformxe2x80x94at least in sectionsxe2x80x94in the direction of flow through the compensation chamber. When flow guiding elements are used, a division of the compensation chamber into partial chambers can also be dispensed with.
With such a design the flow guiding element permits a targeted action on critical portions of the flow in the buffer tank, i.e. on those zones in which separation tendencies are observed or in which the speed profile over the flow cross-section is very irregular, i.e. exhibits great differences in speed.
For influencing a critical flow region inside the compensation chamber in a targeted manner, only a small flow guiding element is needed. The constructional size of the flow guiding element can be determined by one skilled in the art in simple experiments, i.e. adapted to the respective individual case. Starting from a specific size of the flow guiding element with which a uniform flow is achieved in a reliable manner in a selected critical region and the occurrence of irregular speed distribution or separation is reliably avoided, the flow guiding element is made successively smaller. With each new constructional size of the flow guiding element, it is determined numerically or experimentally whether the desired flow-optimizing effect is still achieved. The minimum size of the flow guiding element is the constructional size at which a flow optimization is still observed, i.e. a flow that is made uniform, as well as the prevention of separation and recirculation areas in the selected critical region. The constructional sizes obtained through such test series for the flow guiding element are surprisingly small and will not hinder the core flow through the compensation chamber to any significant extent because they only occupy a small part of the flow cross-section.
Thus, in contrast to the static mixers known from the prior art, which entirely fill the compensation chamber and act on the whole flow through the buffer tank, there is only a locally defined action on the flow according to the invention. The flow guiding element required for such a local action on the flow and for flow optimization is quite small in comparison with static mixers. Thus, upon use of the flow guiding elements of the invention the constructional size of the buffer tank remains substantially unchanged.
The recovery of flow losses achieved by avoiding recirculation zones and dead water zones are in the order of the flow resistance of the flow guiding element, so that the flow resistance of the buffer tank is not increased. Sometimes a reduction of the flow resistance of the buffer tank is even possible.
In the region of separation, in the recirculation zones and the dead water zones, the speed considerably decreases in comparison with the core flow of the polymer melt through the compensation chamber and may even assume negative values. A flow opposite to the flow through the buffer tank, a so-called reverse flow, will then be observed locally.
According to a particularly advantageous development of the invention the flow guiding element may be arranged near a region of the compensation chamber in which a separation or a strong drop in speed occurs in the polymer melt flow. Without a flow guiding element installed in the compensation chamber, these regions can readily be detected by one skilled in the art either experimentally or numerically. When the flow guiding element is placed in the compensation chamber, a flow optimization is effected by the same in a targeted manner in these regions.
In particular in regions in which the flow lines, i.e. the extension of the flow through the compensation chamber, has a strong curvature, there is the risk of separation and of the formation of large vortices in the case of a laminar flow. Therefore, in a further advantageous development, the flow guiding element may be arranged in a region of the compensation chamber in which the flow through the compensation chamber has a strong curvature. The flow guiding elements may be made integral as separate members secured within the compensation chamber, or to the walls of the compensation chamber, as part of a housing or piston of the buffer tank.
Other flow conditions that pose problems for a uniform flow through the compensation chamber may ensue if the compensation chamber has a flow cross-section forming at least one corner. In such a corner a delayed corner flow may form during flow through the compensation chamber. To accelerate the corner flow, thereby avoiding an excessively long residence time of the polymer melt in the corners, the flow guiding element may be arranged in the area of the corner in a further advantageous development. In this arrangement the corner flow forming during flow through the compensation chamber is made uniform by the flow guiding element. This can e.g. be accomplished by the measure that the corners are provided with a flow guiding element which rounds off the corners of the compensation chamber in a manner of benefit to the flow.
The areas in which the polymer flows into and out of the compensation chamber may in particular effect an irregular flow in cases where the polymer melt entry opening and the polymer melt exit opening are arranged in the direction of the axis of symmetry of the annular chamber. In such a case a wall of the compensation chamber which is directly opposite to the polymer melt entry opening is then flown at directly by the polymer melt flowing into the compensation chamber, which results in an irregular speed profile in the area of the polymer melt entry opening. In the outflow area out of the compensation chamber, the flows from the two partial chambers impinge on one another and must be deflected. To optimize the flow in such cases, the flow guiding element may be arranged according to a further advantageous development on the wall of the compensation chamber that is opposite to the polymer melt entry opening and/or the polymer melt exit opening.
The flow guiding element may substantially assume the shape of a preferably radially symmetrical nose oriented towards the polymer melt entry opening. The flow from the polymer melt entry opening into the compensation chamber is divided by said nose in a manner of advantage to the flow and is guided in the direction of the compensation chamber without separation of the flow of the polymer melt or a recirculation.
Alternatively, or in addition to said development, the flow guiding element in a further advantageous development may substantially have the shape of a preferably radially symmetrical nose oriented towards the polymer melt exit opening. In this development the flow out of the compensation chamber is also guided gradually and without the formation of dead water zones and recirculation zones towards the polymer melt exit opening.
In a development of the buffer tank according to the invention the nose may be arranged on the piston.
The flow guiding element may be designed in the form of a blade or a wing in a further advantageous development. These developments of a flow guiding element can influence the flow through the buffer tank in critical areas in a targeted manner. The flow guiding element may e.g. be designed as a flow guiding blade. Such a flow guiding blade guides the rapid core flow into areas of a slow flow or produces, in its trail, a vortex which thoroughly mixes the flow and makes the same uniform.
The present invention also relates to a kit for an extrusion system consisting of an extrusion machine for extruding the polymer melt and of at least one pump means for conveying the polymer melt to the extrusion machine.
The above-mentioned object is achieved in such a kit with a buffer tank according to one of the aforementioned advantageous developments.
In particular, the extrusion machine may be a spinning machine for spinning the polymer solution. In this instance the polymer melt may be a spinning solution, in particular a cellulose solution with NMMO.
To ensure a uniform supply of the polymer melt to the at least one pump means, it is of advantage when the buffer tank is arranged in the conveying direction of the polymer melt in front of said at least one pump means.